drm/amd/display: Wait for hubp read line for Pollock

drm/amd/display: Wait for hubp read line for Pollock

[Why]
Underflow occurred while hubp ret pipe read is idle and the
second pipe is powered up and added. Flickering and underflow
are only observed on Pollock.

[How]
Check the hubp ret pipe read prior to unlock pipes.

Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Agustin Gutierrez <agustin.gutierrez@amd.com>
Signed-off-by: Becle Lee <becle.lee@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>